The conversion of methanol to hydrocarbon products may take place in a fluidized bed process as described, for example, in U.S. Pat. Nos. 4,071,573 and 4,138,440, or in a fixed bed as described in U.S. Pat. Nos. 3,998,899, 3,931,349 and 4,035,430. In the fixed bed process, the methanol is usually first subjected to a dehydrating step, using a catalyst such as gamma-alumina, to form an equilibrium mixture of methanol, dimethylether (DME) and water. This mixture is then passed over a conversion to the hydrocarbon products which are mainly in the range of light gas to gasoline. The water may be removed from the methanol dehydration products prior to conversion to hydrocarbons as may the methanol which can be recycled to he dehydration step, as described in U.S. Pat. No. 4,035,430. Removal of the water is desirable because the catalyst may tend to become deactivated by the presence of the water vapor at the reaction temperatures employed, but this step is by no means essential.
In the operation of the fixed bed process, a major problem which has to be dealt with is the thermal balance. Although the conversion reaction is highly energy efficient, the conversion of the oxygenated feed stream (methanol, DME) to the hydrocarbons is a strongly exothermic reaction liberating approximately 1480 kJ. (1400 Btu) of heat per kilogram of methanol. In an adiabatic reactor this would result in a temperature rise which would lead to extremely fast catalyst aging rates or even to damage to the catalyst. Furthermore, the high temperatures which might occur could cause undesirable products to be produced or the product distribution could be unfavorably changed. For instance, it is known that increases in reactor temperature result in a reduction in gasoline yields, albeit at an increase in gasoline octane. Therefore, close control of the conversion temperature has been found to be highly important to avoid a loss in gasoline yields. A degree of control over the temperature of the catalyst bed can be achieved by suitable choice of bed configuration but this expedient is generally insufficient by itself and other methods must be employed. One particularly efficacious method is to employ a light gas portion of the hydrocarbon product as recycle, as described in U.S. Pat. No. 3,931,349, which patent is incorporated herein by reference in its entirety.
Recycling of process gas limits the temperature rise across the catalyst bed to less than 94.degree. C. (200.degree. F.). Also during the reaction, a small amount of hydrocarbon is deposited on the catalyst as coke, requiring periodic catalyst regeneration. Operation of the process, however, is continuous because additional reactors, arranged in parallel, permit an individual reactor to swing from operation to regeneration. The final gasoline yield from the fixed bed process, after alkylating the light olefins formed, is about 85-90 percent by weight of the total hydrocarbons formed. The remaining hydrocarbons are available, mostly as liquid petroleum gas (LPG) and a small amount of fuel gas.
The conversion of oxygenates is described in depth by C. D. Chang, Catal. Rev.-Sci. Eng., 25, 1(1983) and in U.S. Pat. No. 4,404,414 to Penick et al. These references are incorporated herein in their entirety.
The continuing effort of research workers in the MTG field to improve the process have focused on improvements in gasoline yield as one opportunity. The MTG process is noteworthy for producing good yields of gasoline, above 80% on a cycle average basis. Yet, more noteworthy is the fact that the yields are of an aromatic-rich gasoline with good octane number values. The research challenge, then, is to improve process average cycle yields of gasoline, but without sacrificing gasoline quality as measured by octane number. Investigations heretofore have established what appeared to be inextricable relationships between gasoline yield and octane number such that changes in process conditions leading to an improvement in average gasoline cycle yield were achieved at the expense of product quality as represented by octane number.
In addition to the management of the high exotherm associated with the conversion of oxygenates to gasoline, another factor that restricts and complicates research toward improvements in gasoline yield for the MTG process is the requirement that the process operate at or near quantitative oxygenates conversion. Less than quantitative conversion, referred to as "methanol breakthrough," presents severe problems in waste disposal and/or methanol recovery from aqueous streams, which quickly leads to punishing economic penalties for the process and, therefore, is to be avoided. Accordingly, whatever advances research workers are to make in MTG process yield improvement must be made while maintaining essentially quantitative conversion of methanol.
Research leading to the discovery of the present invention has been directed to programming the temperature of the conversion reaction. In U.S. Pat. No. 4,387,263 to Vogt temperature increasing is used in the conversion of methanol to olefins. The patent is directed to a process for making C.sub.2 to C.sub.4 olefins from gas mixtures containing methanol, dimethylether and optionally steam in the presence of catalysts at temperatures of 250.degree. to 500.degree. C. under pressures of 0.1 and 6 bars. This patent further describes controlling the temperature during the conversion reaction of the methanol-containing feedstock to assure a constant conversion of at most 80% conversion of methanol to hydrocarbons by increasing the temperature during the conversion process to compensate for loss of catalytic activity over time onstream. One example in this patent states that the initial conversion temperature is about 280.degree. C. and is increased up to 360.degree. C. during the gradual loss of catalytic activity. However, although this patent generally discloses increasing the temperature of the feed in the conversion zone in a process seeking olefins to insure a constant conversion proportion of methanol to hydrocarbons, this type of process is quite different from the improved MTG process of the present invention. that is, the MTG process operates at much higher pressrres, i.e., about 10-40 bars, and operates at a 100% conversion level. Moreover, U.S. Pat. No. 4,387,263 seeks an entirely different product,i.e., olefins, than the gasoline boiling range materials of the MTG process. Thus, the type of temperature control used to compensate for loss of catalytic activity in this patent would in no way suggest that temperature programming the MTG conversion reactor inlet temperature would significantly improve cycle average gasoline yield in the MTG process without substantial reduction in cycle average octane number.
It is one object of the present invention to significantly improve the cycle average gasoline yield in the MTG process.
Another object of the present invention is to significantly improve the life of the conversion catalyst in the MTG process, without suffering a substantial reduction in cycle average gasoline octane number.
A further object of the present invention is to improve the MTG process from an economical standpoint by improving cycle average gasoline yields without a concomitant reduction in cycle average octane number.